Heavy metal-contaminated waste materials, such as those found in disposed industrial wastes, contaminated soil, and the like, are often contaminated with strong acids (e.g., H2SO4) having pH values of approximately 1.0 or less in associated pore water. This subsurface acidic pore water can be neutralized in situ by direct injection of an alkali. Neutralization controls mobility of heavy metals in the waste materials via ground water flow through the waste.
Typical in situ neutralization methods involve injecting an acid neutralizing agent comprising carbonates, such as Na2CO3, NaHCO3, CaCO3, MgCO3, and the like, in the form of an alkaline solution or slurry. Such processes liberate large quantities of CO2, thereby increasing down hole back pressure and requiring higher injection pressures. The liberated CO2 finds preferential pathways to release a pent up excess subsurface pressure causing “daylighting” of the injected material.
To avoid daylighting issues associated with carbonate alkali injection, non-carbonate alkaline agents such as NaOH, Ca(OH)2, and the like can be injected to neutralize the acidic pore water. However, injection of substantial quantities of strong alkaline agents can be problematic in that the pore water turns rapidly from strongly acidic to strongly alkaline. This is particularly undesired because a high pH increases solubility of emphoteric heavy metals and therefore increases the potential mobility of such metals. Alternative approaches to neutralizing subsurface acidic pore water in situ are desired.